Loop antennas exhibit relatively narrow bandwidth and typically are tuned to a desired transmit and/or receive frequency by a variable capacitor. Prior art loop antennas that operate at relatively high power levels typically employ either an air variable or vacuum variable capacitor as the antenna tuning element.
Although commercially available air and vacuum variable capacitors provide satisfactory operation in some loop antenna applications, both types of capacitors often present disadvantages from the standpoint of size, weight and cost. In this regard, because the dielectric constant of air is relatively low, air variable capacitors suitable for tuning a loop antenna typically include a plurality of relatively large metal plates that are spaced apart from one another by a substantial distance in order to provide the required capacitance range and voltage breakdown rating. Although vacuum variable capacitors typically are smaller in size than comparable air variable capacitors, the cost is higher and other disadvantages are present. One factor that contributes to the higher cost of vacuum variable capacitors is the need to maintain satisfactory vacuum seals. Further, the vacuum variable capacitors typically use capacitor plates that are precisely machined and closely spaced from one another, thereby increasing cost. Moreover, because of the vacuum seal and close fitting capacitor plate structure, vacuum variable capacitors are relatively delicate.
In addition to cost and size disadvantages, commercially available air and vacuum variable capacitors often do not provide adequate voltage breakdown ratings for loop antennas that are used with high power transmitters. In this regard, when an air variable capacitor is used for tuning a loop antenna, the capacitor often must be operated below its voltage breakdown rating to allow for high ambient humidity and/or operation at high altitude. Vacuum variable capacitors can be especially disadvantageous in high power loop antenna applications. Specifically, interplate arcing that occurs upon voltage breakdown can vaporize affected portions of the capacitor plates thereby resulting in "barnacle" growth that permanently damages (or even destroys) the capacitor.
Moreover, achieving maximum antenna efficiency (low power loss) often is difficult with a loop antenna that employs a conventional air or vacuum variable capacitor. Specifically, the radiation resistance of a small loop antenna often is on the order of 0.1 ohms. Thus, high currents flow through the antenna structure during high power transmission. Accordingly, to prevent excessive loss of power (loss of antenna efficiency), the current path through the entire antenna must be very low in resistance.
Prior art loop antennas typically are formed of a rigid metal conductor and are in the shape of a square or other polygon (e.g., a hexagon). In such an arrangement, the air variable or vacuum variable antenna tuning capacitor typically is mounted in a gap or opening in one arm of the antenna. To minimize ohmic contact resistance (and hence power loss), the tuning capacitor generally includes relatively large electrical terminals that often are machined for flatness and may be gold or silver plated to minimize junction resistance when the capacitor terminals are bolted or otherwise connected to the antenna. Although these measures are helpful in minimizing junction resistance, the resistance and resulting power loss often is higher than desired. Further, machining and/or plating capacitor terminals further increases the cost of the tuning capacitor. Moreover, in situations in which the antenna is exposed to the environment, corrosion and loosening of the antenna-capacitor interconnections may occur thereby increasing the junction resistance. Even further, air variable and vacuum variable capacitors include internal electrical junctions, e.g., electrical connections between the capacitor plates and mounting elements within the capacitor. These internal electrical junctions present additional ohmic contacts that cause power loss and, hence, further decrease the efficiency of the antenna.
There is yet another disadvantage or drawback associated with using an air variable or vacuum variable capacitor as the tuning element of a loop antenna. Specifically, relatively high torque usually is required to tune air variable and vacuum variable capacitors that are suitable for use with a loop antenna that operates at high power levels. In the prior art, relatively complex control units that often include a stepping motor, a gear reduction unit and associated circuitry have been used to supply the torque required to drive the tuning capacitor. These control units not only increase the cost of the antenna system, but cannot rapidly tune the antenna to a desired frequency. Because of this tuning limitation, prior art loop antennas have not been suitable for use in applications such as secure communications systems that use burst-mode or frequency hopping techniques.